Field of the Disclosure
The present invention relates to method for the quantitative determination of nonisothermal thermooxidative degradation effects of a polyolefin material comprising a residual catalyst. The method utilizes thermogravimetric analysis (TGA) and mathematical modeling with a rigorous constitutive kinetic model.
Description of Related Art
Ethylene homopolymers are commercially produced using supported Ziegler-Natta, chrome, and metallocene catalysts. The as-synthesized catalysts disintegrate during polymerization, architect the molecular structures of the polymer backbones, and develop the desired particulate resin morphology (Webb S W, Weist E L, Chiovetta M G, Laurence R L, Conner W C (1991) The Can J Chem Eng 69:665-681; Grof Z, Kosek J, Marek M (2005) AIChE J 51:2048-2067; Zheng X, Smit M, Chadwick J C, Loos J (2005) Macromolecules 38:4673-4678; Martino A D, Broyer J P, Spitz R, Weickert G, McKenna T F L (2005) Macromol Rapid Commun 26:215-220; Martino A D, Weickert G, Sidoroff F, McKenna T F L (2007) Macromol React Eng 1:38-352; Silva F M, Broyer J P, Novat C, Lima E L, Pinto J C, McKenna T F L (2005) Macromol React Eng 16:1846-1853; Atiqullah M, Akhtar M N, Moman A, Abu-Raqabah A H, Palackal S J, Al-Muallem H A, Hamed O M (2007) Appl. Catal. A: General 320:134-143; Atiqullah M, Moman A, Akhtar M N, Al-Muallem H A, Abu-Raqabah A H, Neaz A (2007) J Appl Poly Sci 106:3149-3157—incorporated by reference in its entirety). The homopolymer molecular structures are characterized by molecular weight distributions (polydispersity index) and their averages, and side chain branching (if any). The following factors—support type, catalyst precursors, and the surface chemistry and structure of the as-synthesized catalyst—affect the morphology and molecular structures of the resulting polymers.
After polymerization, the as-synthesized catalysts remain as residues in the final polymer. Being present in trace amounts, the residual catalysts are not recovered and recycled. Therefore, they become an integral part of the matrix of the synthesized polymer, and eventually they affect polymer processing, as well as applications and properties of the end-products. The concentration and distribution of the residual catalysts depend on the support type, catalyst activity, and polymerization process types and reaction conditions.
FIG. 1 illustrates how the disintegration of a supported catalyst particle generates the residual catalysts through macro-scale to micro-scale to molecular-scale events. The residual catalysts are functionalized molecular entity (Dompazis G, Kanellopoulos V, Chatzidoukas C, Kiparissides C (2005) Proceedings, 3rd European Conference on the Reaction Engineering of Polyolefins, Lyon, June 20-24, France—incorporated by reference in its entirety), and differ in surface chemistry and structure from the corresponding as-synthesized catalysts. Residual catalysts structure as used herein refers to the catalyst transition metal, combined with the surrounding ligands including the disintegrated support. The metal center experiences electronic and steric effects due to its Lewis acidity (electron density) and the characteristic configuration of the surrounding ligands (Lee I-M, Gauthier W J, Ball J M., Iyengar B., Collins S (1992) Organometallics 11:2115-2122; Rappé A K, Skiff W M, Casewit C J, (2000) Chem Rev 100:1435-1456; Atiqullah M, Akhtar M N, Faiz M, Moman A, Abu-Raqabah A H, Khan J H, Wazeer M I (2006) Surf Inter Anal 38:1319-1327; Kong Y, Yi J, Dou X, Liu W, Huang Q, Gao K, Yang W (2010) Polymer 51:3859-3866—incorporated by reference in its entirety).
The ethylene homopolymers, including HDPEs, have a large number of applications. Therefore, in recent years, the production and consumption of these thermoplastics have significantly increased. This is a boon. However, what is bad is that they are flammable; they hardly degrade in the ambient atmosphere. Consequently, they are an obnoxious source of environmental pollution, causing littering and accumulation in the municipal solid waste. Hence, it is a subject of serious ecological concern and issue (Camacho W, Karlsson S (2002) Polym Degrad Stab 78:385-391—incorporated by reference in its entirety). Stringent legislations are, therefore, being developed worldwide to effectively address these problems. Conversion of the post-use polyolefin thermoplastic end-products into value-added hydrocarbons is being practiced as a prospective solution. Therefore, research continues in this area that involves catalytic cracking of commercial polyolefin resins (Marcilla A, Hernández M D R, García Á N (2007) J Anal Appl Pyrol 79:424-432; Fernandes V J, Araujo A S, Fernandes G J T (1999) J Ther Anal Calorim 56:275-285; Uemichi Y, Kashiwaya Y, Ayame A, Kanoh H (1984) Chem Lett 1:41-44; Uemichi Y, Kashiwaya Y, Tsukidate M, Ayame A, Kanoh H (1983) Bull Chem Soc Jpn 56:2768-2773—incorporated by reference in its entirety).
To the best of our knowledge, the role of residual catalysts, especially in the high temperature thermooxidative degradation of ethylene homopolymers, has not yet been studied even though this can provide important information on material behavior under more realistic atmospheric conditions. The average diameter of residual catalyst particles may be in the order of 100 nm (Ben G S, Goss H, Nakatani, Graeme A, Georgeb M, Terano (2003) Polym Degrad Stab 82: 119-126—incorporated by reference in its entirety). Consequently, they are likely to interact on a molecular level with the polymer backbones. Hence, they may act as a source of temperature-resistant heterogeneous catalytic moiety, and consequently interfere into the overall thermooxidative degradation process.
To study the role of the residual catalysts in high temperature thermooxidative degradation is a pre-requisite to fully understanding catalytic cracking of polyolefins.